A transmission belt is generally produced by: winding, on an outer periphery of a cylindrical mold, at least a first unvulcanized rubber sheet, a core wire and a second unvulcanized rubber sheet in this order, to thereby form a laminate; vulcanizing the laminate to form a vulcanized belt sleeve; and cutting the vulcanized belt sleeve into round slices in a circumferential direction. A glass core wire, a polyester core wire and the like are used widely as the core wire, but those core wires cannot satisfy both of high strength and elasticity. For this reason, use of an aramid core wire is desired. However, even though the aramid core wire is used, elasticity is not sufficient as compared with the polyester core wire, and a single yarn may project on a cut surface (side surface of a transmission belt) of a vulcanized belt sleeve and fray is easy to occur. When the transmission belt has ran in the state that fray occurred, the core wire pops out (is pulled out) and is tangled in a pulley, and the transmission belt is broken. As a result, durability of the transmission belt is remarkably deteriorated. For this reason, fray of a single yarn on a cut surface of the transmission belt must physically removed by polishing or the like, and such a work greatly decreases productivity. Furthermore, in the case of the aramid core, even though a single yarn does not project on a cut surface of the transmission belt before running, when the transmission belt runs, the single yarn becomes to project and fray is easy to occur. To reduce those disadvantages, a method for preventing fray of a single yarn by treating an aramid core wire with a specific treating agent is proposed.
For example, Patent Document 1 proposes a method comprising winding a cord for a belt such as an aramid fiber cord on an outer periphery of a permeation tube having many discharging holes provided on the outer periphery thereof, and discharging a resorcin-formaldehyde-rubber latex (RFL) liquid through the discharging holes from the inside of the permeation tube in the state of dipping the cord for a belt wound around the permeation tube in the RFL liquid, thereby permeating the RFL liquid in the cord for a belt. However, in this method, bundling property of aramid fibers is not yet sufficient, and fray of a single yarn cannot effectively be prevented.
Patent Document 2 describes a method comprising: imparting a treating agent comprising an epoxy compound and a rubber latex, which are mixed in specific proportions, to a substantially non-twisted aromatic polyamide fiber; heat-treating the fiber under specific conditions; twisting the fiber in a specific twist coefficient; and applying an adhesive to the fiber. Patent Document 3 describes a method comprising: impregnating non-twisted or soft-twisted filaments constituted of organic fibers such as aramid fibers in which molecular chains are oriented in a fiber longitudinal direction, with a pretreatment agent comprising an epoxy resin or an isocyanate resin as a main component, optionally followed by heat treatment, to form a cord raw material; impregnating the cord raw material with an RFL liquid, optionally followed by heat treatment; and arranging and twisting one or more treated cord raw materials. Patent Document 4 describes a method comprising: previously dipping aramid fibers in a treating liquid containing a polyepoxide compound having two or more epoxy groups in a substantially non-twisted state; heat-treating the aramid fibers under specific conditions; twisting the aramid fibers in a specific twist constant to form an aramid fiber cord; impregnating the aramid fiber cord with a treating liquid containing RFL under vacuum and pressurized conditions; heat-treating the aramid fiber cord; and further treating the aramid fiber cord with the treating agent containing RFL. However, in those methods, the aramid fiber cord becomes too hard, resulting in decrease in bending fatigue resistance of a transmission belt.
Patent Document 5 proposes a method comprising: treating a core wire for a transmission belt, comprising twisted yarn cords with a pretreatment liquid containing a nitrile rubber-modified epoxy resin and an alkyl phenol-formaldehyde resin; subsequently treating the core wire with an RFL liquid comprising a nitrile rubber latex or a hydrogenated nitrile rubber latex; and overcoat-treating the core wire with a rubber cement prepared by dissolving a nitrile rubber blend or a hydrogenated nitrile rubber blend in a solvent. Patent Document 6 describes a method comprising: dipping an aramid fiber cord in a first treating liquid containing an aqueous mixed liquid of an initial RF condensate and rubber latex and an aqueous dispersion of a substantially water-insoluble epoxy resin that is a solid at normal temperature and has an epoxy equivalent of 300 or less; drying the aramid fiber cord having the first treating liquid adhered thereto at specific temperature; heat set-treating the aramid cord at specific temperature; dipping the heat set-treated aramid fiber cord in a second treating liquid containing an initial RF condensate and a rubber latex containing a specific proportion of an acrylonitrile-butadiene rubber; drying the aramid fiber cord having the second treating agent adhered thereto at specific temperature; and subjecting the aramid fiber cord to a normalization treatment at specific temperature, in order to improve adhesiveness between a aramid fiber cord and a rubber composition in rubber products such as a pneumatic tire and a hose. However, those methods use a water-insoluble epoxy resin, and the aramid fiber cord cannot be impregnated with the epoxy resin up to the inside thereof. As a result, fibers cannot sufficiently be bundled, and a single yarn of the aramid fiber cord cannot effectively be prevented from fraying.
Thus, trial and error are being made to satisfy both of fray resistance and bending fatigue resistance of an aramid core wire over about 20 years from the start to use the aramid core wire, but an aramid core wire satisfying both characteristics is not yet obtained.    Patent Document 1: JP-A-2009-74210    Patent Document 2: JP-A-1-207480    Patent Document 3: JP-A-6-57568    Patent Document 4: JP-A-8-100370    Patent Document 5: JP-A-2001-322184    Patent Document 6: JP-A-2006-45703